Does air have pressure? How do we know air pressure? Easy experiments that prove that air has pressure.
You can easily understand why water has pressure, because water is a fairly dense liquid. But because air is a mixture of gases far less dense than water, you may not expect air to have much pressure, if any. A cubic foot of water weighs 62.4 pounds. A cubic foot of air weighs only about 1/4 ounces or .08 pound. So water is 800 times as dense as air. If you go down more than a few feet under water, you feel its pressure at once. Yet people seldom notice the pressure of air, which is acting on you all the time.
A mile or more below the surface of the sea, the pressure of the water is over a ton per square inch. This tremendous pressure does not affect the fish that live there any more than the pressure of the air affects you. The fish grew under the pressure of the water. The pressure on the inside of their bodies is the same as the pressure on the outside. Your body has grown under the pressure of the air. The pressure on the outside is balanced by an equal pressure on the inside of your body.
You have lived all your life at the bottom of the ocean of air that surrounds the earth. This ocean of air, called the atmosphere, is at least 500 miles deep. And gravity is pulling down on every cubic foot of air in the atmosphere just as it pulls down on every cubic foot of water. The deeper the water gets, the greater its pressure becomes. The same is true of air. The deeper the atmosphere is, the greater its pressure becomes. So even though the density of air is very little, there is enough air straight up above you to produce much pressure at the earth’s surface. You do not feel this pressure because this pressure is balanced by an equal pressure on the inside of your body. In other words, your body has adjusted itself inside and outside as you grew up. But air really exerts a great force on you and everything around you.
A famous experiment showing that air has pressure was first done about 300 years ago at Magdeburg, Germany, by a scientist named Otto von Guericke. He used two metal half spheres, or hemispheres, 22 inches in diameter. The rims of the hemispheres were fitted tightly together so that air could not enter. The air already inside the hemispheres was then pumped out of the spheres. Then the air on the outside pressed inward with so much force that the hemispheres could not be pulled apart until eight horses had been hitched to each one.
To find out how hard gravity makes air press on things, we need some way of measuring the weight of a column of air above us. We cannot use a scale to weigh this column, but we can use a kind of automatic balance called a barometer. A barometer is an instrument for measuring the pressure of the atmosphere.
Gravity is pulling down on the air above the open dish and also on the mercury in the glass tube. The air presses down on the mercury in the dish. But it does not press down on the mercury in the tube, because the top of the tube is closed. The mercury inside the tube sinks until its pressure just balances the pressure of the air outside. The pressure of the column of mercury is now equal to the pressure of the atmosphere. The top of the column stands about 30 inches above the mercury in the dish. So if you can figure out how much pressure 30 inches of mercury have, you will know how hard the atmosphere is pressing on the earth.
The pressure of the atmosphere is not always the same everywhere. If you carry a barometer up in the elevator of a tall building, the mercury sinks a little inside the glass tube. As the elevator goes down, the mercury rises again. This shows that air pressure changes as you go up and down. From what you have learned about water pressure, you should be able to understand why this happens. The pressure becomes greater as the water gets deeper. The same thing is true of the pressure of the atmosphere. When you are down low, you have more air above you. The air pressure is greater. When you are up high, you have less air above you. Then the air pressure is less.